iio: multiplexer: iio-mux: Support settle-time-us property

iio: multiplexer: iio-mux: Support settle-time-us property

If the devicetree specifies that the hardware requires a settle time,
pass this time on to the mux APIs.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20211007134641.13417-4-vincent.whitchurch@axis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>